The invention relates to heat exchangers which use heat exchange assemblies based on a circulating fluid and which are designed to be employed in a severe temperature environment.
Particular, but non-limiting, fields of application of the invention are systems for transforming materials, e.g. nuclear fusion reactors, and propulsion systems, in particular wall elements for the combustion chambers of jet engines, in particular ram jets.
The heat exchangers used in such applications are generally made of metal, at least in part. Unfortunately, the thermal and mechanical properties of metals and metal alloys limit their field of use, and also their performance and safety. Furthermore, metal heat exchangers are heavy and bulky, which penalizes use thereof, at least in some applications.
It has been envisaged to use refractory composite materials alone or in combination with metals for making heat exchangers designed to be used in a severe temperature environment, in particular for the wall of a nuclear fusion reactor. Thus, patent application WO 98,/03297 describes making such a heat exchanger by brazing pieces of carbon-carbon (C/C) composite material on a metal (copper) substrate cooled by fluid circulation. That involves using a metal. Also known is U.S. Pat. No. 5,583,895 which describes a heat exchanger structure for the same application in the form of a C/C composite material block in which fluid circulation passages are formed. The walls of the passages are made leakproof by a metal lining, e.g. made of copper, which is brazed to the C/C composite material.
An object of the invention is to provide a heat exchanger capable of being used in a severe temperature environment.
Another object of the invention is to provide a heat exchanger in which the various thermal and structural functions can be optimized so that mass, bulk, and cost are reduced as far as possible.
Another object of the invention is to provide a heat exchanger capable of being made easily.
Another object of the invention is to provide a method of manufacturing such a heat exchanger.
A heat exchanger of the invention is characterized in that it comprises an intermediate portion of refractory composite material in which fluid circulation channels are formed, the intermediate portion being interposed between a portion of refractory composite material forming a heat shield and a portion of thermostructural composite material forming a support structure of the heat exchanger, the portions constituting the heat exchanger being assembled together by brazing.
A thermostructural composite material is a composite material having mechanical properties making it suitable for constituting structural elements and which conserves these properties up to high temperatures. Such thermostructural composite materials are typically composite materials having fiber reinforcement of refractory fibers such as carbon fibers or ceramic fibers, densified by a refractory matrix such as a carbon matrix or a ceramic matrix. Examples of thermostructural composite materials are carbon/carbon (C/C) composite materials with reinforcing fibers and a matrix made of carbon, and ceramic matrix composite (CMC) materials, e.g. having a matrix of silicon carbide (SiC).
Advantageously, the thermostructural composite material forming the support structure of the heat exchanger is a C/C composite material. It can be in the form of a honeycomb or of a composite material in which the fiber reinforcement is formed by superposed layers of fibers bonded together by fibers extending transversely relative to the layers, as can be obtained by needling, e.g. as described in patent U.S. Pat. No. 4,790,052.
Also advantageously, the material of the intermediate portion is also a C/C composite material which is then used more for its refractory qualities than for its structural qualities.
It is possible to envisage making the portion that forms the support structure and the intermediate portion as a single block of C/C composite material to which the portion forming the heat shield is brazed.
Also advantageously, the material of the portion forming the heat shield is a material of the CMC type, e.g a C/SiC or SiC/SiC composite material (i.e. a material having reinforcing fibers of carbon or of silicon carbide and densified by a silicon carbide matrix), which materials are more suitable than C/C composite materials for exposure to intense heat flux, particularly in an oxidizing atmosphere. An advantage of the heat exchanger of the invention lies in the possibility of selecting materials that are the most suitable for performing the thermal functions and for performing the mechanical functions of the heat exchanger, thereby making it possible to optimize manufacture of the heat exchanger in terms of performance and bulk.
According to yet another feature of the heat exchanger of the invention, the fluid circulation channels are formed in one face of the intermediate portion, e.g. by machining, and they are defined in part by the adjacent wall of one of the other two portions. The fluid circulation channels are thus particularly simple to make.
If necessary, the channels can be made leakproof by forming a coating on their walls, e.g. a thin layer of metal coating. Such a coating can be formed over the entire facing faces of the portions that are to be assembled together so as to facilitate brazing, thereby also constituting an adhesion layer for brazing purposes.